What next for the International Space Station?

It's a challenging time for the International Space Station (ISS). The single most expensive engineering project in human history and one of the most complex pieces of machinery ever assembled, the future of the ISS remains uncertain after the ending of the Space Shuttle program and the grounding of Russia's Soyuz fleet following an accident last month. While the recently announced resumption of manned Soyuz flights means the danger of the station being evacuated and mothballed has receded ... it hasn't ended.

These days, outer space looks like a shooting gallery, and the uncontrolled re-entry of the American Upper Atmosphere Research Satellite (UARS) on September 24th, followed by the impending plunge of the defunct German 2.4-ton space telescope Röntgensatellit (ROSAT) - expected in October - shows that space debris is a hazard to those on the ground as well as to orbiting satellites. Weighing over 200 times more than ROSAT, the ISS is like Thor's hammer compared to a feather, and keeping the station operational and under complete control is the top safety priority of NASA and the other ISS partner space agencies. This is one major reason why mothballing the station is something to be avoided and why contingency planning is so important. Unmanned, the danger of the ISS turning into an uncontrolled missile increases greatly.

The safe return of three astronauts from the ISS on September 16, 2011 caused the world to breathe a sigh of relief. As NASA astronaut Ron Garan and Russian cosmonauts Andrei Borisenko and Alexander Samokutyayev climbed out of their Soyuz TMA-21 capsule onto the steppes of Kazakhstan, it seemed as though the future of the ISS had been assured. The Russians had investigated the August crash of an unmanned Progress cargo rocket bound for the ISS, and had announced that the cause had been identified and manned flights of the Soyuz spacecraft to the station would resume in November. However, the crash has already had a significant impact on the station's schedule and the possibility of the station being evacuated remains a threat to the program's future.

The result of a partnership of 15 nations, the International Space Station cost in excess of US$100 billion, weighing 990,000 pounds (450,000 kg), covering an area the size of a football field and having the volume of a 747 Jumbo Jet. This complex engineering marvel is also designed to host a human crew, which imposes an extremely high standard of performance and reliability within very narrow engineering limits, and it is designed to be assembled and maintained by that crew.

This means that the ISS requires a margin of safety so high that there isn't anything to compare it to outside of a nuclear reactor. It also means that there are many opportunities for Murphy's Law to rear its ugly head, and makes the station very dependent on the crew who look after it.

Progress cargo rocket crisis

The crash of the Progress cargo rocket was a major crisis. Both the Soyuz spacecraft and the Progress share the same Soyuz launch vehicle, and the fault in the third stage that caused the crash would have been common to both. Even though the cause was eventually identified as a loss of pressure in a turbo pump, the crash has already cost the ISS three tons of supplies, and the investigation has already resulted in the scratching of the September 22 Soyuz launch that would have sent three new crewmen to the station to replace those who returned. This means that the ISS has only half the crew required for full operation. In addition, the private sector cargo ship

SpaceX Dragon, which is currently scheduled to dock with the station in December could face

delays, and there have recently been conflicting reports about whether it will be allowed to dock when the test is finally carried out.

Then there is the matter of the Soyuz itself. Resuming flights to the ISS is more than a matter of sending up another crew. An unmanned Soyuz capsule has to be tested first, and that means more expense and delays. This causes a number of problems because the remaining three crew on the station are up against some very hard deadlines. Winter is coming and the hours of daylight available at the Kazakhstan landing area are limited, which is very serious when it comes to organizing recovery operations in the steppe terrain. This is made worse due to the orbit of the station only allowing landings in Kazakhstan between certain dates. This is the reason three of the crew had to return on September 16. Any later, and they would have been stuck on the station until October 27. Also, the Soyuz capsule now docked with the ISS has a service life of only 200 days, which expires on December 24.

Aside from the problems for the crew, having only three men on board means that science experiments take second place to just running the station. While many experiments can run automatically or through ground control, the primary purpose of the station, aside from its construction, will be curtailed. This is exactly the same problem the ISS program faced in 2003 after the Columbia disaster, when the American shuttle fleet was grounded and there weren't enough Soyuz craft available from the cash-strapped Russian space agency to take up the slack.

Of course, the resumption of Soyuz flights this year is only a temporary solution. Even if the Progress was grounded for good, there are at least two other cargo craft from the European Space Agency (ESA) and American private industry available to service the station, and at least three more private freighters are under development. The real problem is that there are no crew-capable craft to replace the Soyuz, and won't be for at least a couple of years. Both the European Space Agency freighter and the private Dragon capsule are under development as manned versions and both are further along than their competitors, but that is no use to the ISS at the moment. Until some alternative to Soyuz becomes available, the ISS is dependent on an aging crew transport system designed in the 1960s, now proven to be of unknown reliability. That sort of all-your-eggs-in-one-basket situation makes engineers and policy makers very nervous.

An unmanned ISS?

But what would happen if the test flight is unsuccessful and the ISS must be evacuated in November? What problems would the station face and what could be done to prevent them?

One thing that would certainly happen is that the departing crew would take steps to minimize risks to the evacuated station. Bulkhead doors would be sealed, conduits closed, systems unnecessary to the basic operation of the station would be shut off, circuit breakers opened, and so on. The idea would be to reduce the chances of something going wrong and to seal off the different parts of the station from one another so that if something did go wrong, the damage would be confined to as small an area as possible.

But what are the potential problems? The official NASA line is that there wouldn't be much of any. The station could be operated from the ground " for several years - attitude rockets could be refueled by unmanned craft and redundant parts could be installed to insure against equipment failures. Without a crew, the atmosphere inside the ISS could be reduced to zero humidity, which would protect the station against condensation and corrosion. There would obviously be a major impact on experiments, with some continuing on automatic and others, especially biological experiments, going on hold.

However, there are others who don't take such a sanguine view of the situation. In an interview with Florida Today, NASA Space Station Program Manager Mike Suffredini said that the risk of losing the station within six months of evacuation was 1 in 10 and after that the odds leap to 50/50. "It's not a trivial thing," Said Mr Suffredni. "If you look at probability risk assessments, some of the numbers are not insignificant. There is a greater risk of losing ISS when it's unmanned than if it were manned."

He went on to say that a systems failure could affect the attitude controls, which would reduce the ability of the station to receive radio signals from ground control.

Dr. Leroy Chiao, former ISS crewman, agrees. Speaking on National Public Radio, Dr. Chiao said that the greatest danger is if the station started to tumble. This would prevent astronauts from returning to the station and would make it impossible to adjust the station's orbit. Every day the ISS loses 500 ft (150 m) of altitude due to drag caused by what little is left of the Earth's atmosphere at its orbital height. Under ordinary conditions, a Progress craft could be docked with the station and use its thrusters to push the station into a higher orbit. Also, steps can be taken to reduce the drag on the station by feathering the solar panels, much as a sailing vessel reefs its sails when the wind blows too hard. However, if docking isn't possible, the orbit will eventually decay and the station would make an uncontrolled re-entry with the impact point being no one knows where.

Another problem that the empty ISS could face would be a loss of temperature control. At any given moment, half of the ISS is in direct sunlight and the other in shadow. The sunward side is heated to 250 degrees F (121 C), while the shadowed side plunges to minus 250 degrees F (-157 C). It's also surrounded by a vacuum, which makes the ISS the world's largest Thermos flask. Should the cooling system fail while the astronauts are away, the interior temperatures could rapidly rise rapidly, making reoccupying the station much more difficult and putting more stress on the systems.

A power failure is another hazard. The station relies on banks of solar panels for the electricity to run all of its systems, including the gyroscopes that prevent it from tumbling. If something happens to knock out that power system or seriously degrade it, the station will face the same danger of becoming unstable as it did in 2004 when gyroscope failures required a series of space walks to effect repairs.

Oddly enough, that is one of the problems that has actually diminished as the ISS nears completion. The station boasts a number of teleoperated robots and the Canadian Space Agency's robot, Dextre, recently carried out a repair job on the outside of the station where it replaced a faulty circuit-breaker box while under ground control from CSA headquarters in Saint-Hubert, Quebec. The ISS crew was unaware of this, as they were asleep at the time. It's some comfort to know that if the station is evacuated and some simple repairs (such as replacing a component package) are needed, the station's answer to R2D2 will be on hand.

Some problems, however, are beyond the scope of robots. A loss of pressure inside one of the modules could have consequences ranging from difficult to catastrophic, depending on where and how the loss of air occurs. A slow leak of air out of one of the modules would hamper returning to the station at the very least. On the other hand, a blown seal or a strike by a meteor or bit of space debris that punctures a module hull could cause a sudden blast of escaping air, that could act like a rocket and cause the station to go into a fatal tumble.

Above all of these possibilities in terms of seriousness is that of fire. The terror of submariners and aircraft crews, the threat of fire in the confines of a spacecraft have haunted NASA and other space agencies ever since the fatal launch pad blaze that killed the crew of Apollo 1 during a ground test in 1967. Since that disaster, NASA has taken strong measures to prevent fires in space (the crew of NASA's first space station, Skylab, complained that the flame retardant in their wash cloths made them worthless), but the hazard has not been eliminated. Unless the atmosphere in the ISS was replaced with carbon dioxide or nitrogen, which is unlikely, the danger of fire would remain after evacuation. On the plus side, having no crew means that no lives would be at stake. On the other hand, without a crew, detecting and fighting fires becomes much more difficult and the chances of at least one module being wrecked increases.

If the Soyuz test is successful and if manned flights resume, then the threat of evacuation for the ISS recedes, but that still depends very much on that very large word "if." Reliance on Soyuz means that even if normal ferry service resumes, evacuation remains a very real possibility. This is especially true in that space still holds many unknowns and surprises. When the last crew left Skylab on February 8, 1974, NASA thought that America's first space station would remain safely in orbit until the mid 1980s and plans were even drawn up for the new Space Shuttle to reclaim it. However, unforeseen massive solar activity increased atmospheric drag and Skylab plunged to Earth in 1979 with fragments crashing in Australia, though without causing any injuries or property damage. That drama was replayed in miniature this month as the American Upper Atmosphere Research Satellite crashed to earth, and it may again if resourcefulness and luck fail the ISS partners.